As it starts up, it only ever gets that far before spontaneously rebooting. It won't even respond to attempts to boot from the CD rom.
I've put a HD from an identical machine into it instead and it works absolutely fine; when I put my HD in as a secondary drive, I can see everything on it absolutely fine, it just won't boot when it's the only HD in the machine.
This started when my Vista install became corrupted (the machine dual-boots into Ubuntu), and the restore process essentially re-isntalled Vista. It worked once, but after I installed a few 'windows updates', it needed a reboot, and it hasn't worked since. Not even into Ubuntu, or booting from CD rom. Short of installing the drive as a secondary one on another machine, downloading all the data off it and reformatting, is there a way to fix this?
EDIT: Just did some more experimenting; nothing I do convinces it to boot from the CD rom, EXCEPT disconnecting that drive altogether. With a different HD in, or no HD at all, it boots from CD rom just fine. With that drive in, even if it's supposed to boot from the CD rom first or if I specifically select the CD rom from the boot menu, it still fails.
Further edit: Well, even unplugging the drive to persuade it to boot from CD first, then plugging the drive in failed. So, with it installed as a secondary HD, I did a low-level format, and it finally decided to allow it to boot from CD. Went through the full 're-installing vista' thing from the recovery CD... and now that it's a bootable HD again, it STILL won't boot from the CD when I ask it to, it just goes straight into the Vista install.